The present invention is generally related to simulcast communications systems and more particularly to an improved simulcast data communications systems.
In the prior art, simulcast systems have been used to substantially simultaneously transmit from multiple base stations identical messages at a particular radio frequency. As a result, the same message can be transmitted simultaneously or "simulcasted" over a very large and diverse geographical area. However, in areas covered by two or more base stations, the message may be degraded by differences in the message path delays, phases, modulation levels, and frequencies of the base station transmitters. Such degradation may be reduced somewhat by accurately controlling the base station transmitter frequencies as described in U.S. Pat. No. 4,188,582, by randomly offsetting the base station transmitter frequencies as described in U.S. Pat. No. 4,570,265, and/or by accurately controlling the message delay from the controller to each base station as described in U.S. Pat. No. 4,255,814. However, such prior art techniques typically require expensive microwave links for connecting the controller to each of the base stations and delay compensation circuitry in each of the microwave links. Accordingly, there is a need for an improved method and apparatus for inexpensively compensating for message delays between the controller and the base stations in simulcast communications systems.